The efficiency of turbine engines, for example gas turbines, is increased as the firing temperature, otherwise known as the working temperature, of the turbine increases. This increase in temperature results in at least some increase in power with the use of the same, if not less, fuel. Thus it is desirable to raise the firing temperature of a turbine to increase the efficiency.
However, as the firing temperature of gas turbines rises, the metal temperature of the combustion components, including but not limited to combustion liners and transition pieces otherwise know as ducts, increases. A combustion liner is incorporated into a turbine, and defines, in part with a transition piece or duct, an area for a flame to burn fuel. These components, as well as other components in the gas path environment, are subject to significant temperature extremes and degradation by oxidizing and corrosive environments.
Turbine combustion components, such as but not limited to, combustion liners, ducts, combustor deflectors, combustor centerbodies, nozzles and other structural hardware are often formed of heat resistant materials. The heat resistant materials are often coated with other heat resistant materials. For example, turbine components may be formed of wrought superalloys, such as but not limited to Hasteloy alloys, Nimonic alloys, Inconel alloys, and other similar alloys. These superalloys do not possess a desirable oxidation resistance at high temperatures, for example at temperatures greater than about 1500° F. Therefore, to reduce the turbine component temperatures and to provide oxidation and corrosion protection against hot combustion gasses, a heat resistant coating, such as but not limited to, a bond coating and a thermal barrier coating (TBC) are often applied to a surface of the turbine component exposed to the hot combustion gases, or otherwise known as a hot side surface. For example, a turbine component may include a thermally sprayed MCrAlY overlay coating as a bond coat and an air plasma sprayed (APS) zirconia-based ceramic as an insulating TBC. Often, the TBC is a zirconia stabilized with yttria ceramic.
Recently, ceramic top coat compositions with low thermal conductivity have increased temperature operation and strained the capability of applying only a thermal barrier coating to the hot side of turbine components. Current TBC systems have performed well in service in certain applications, however, improved coatings are sought to achieve greater temperature-thermal cycler time capability for longer service intervals or temperature capability.
What is needed is a coating system that enhances heat transfer from turbine components allowing turbine components to operate at higher system temperatures.